Injection water viscosifier for enhanced oil recovery

ABSTRACT

High molecular weight N,N-dimethylacrylamide copolymers and terpolymers were synthesized. These polymers offer outstanding advantages as injection water viscosifiers in enhanced oil recovery processes including chemical, miscible, and steam or in processes requiring profile improvement through adsorption and/or gelation. They are very valuable in applications where high salinity is a problem since they are relatively insensitive to metal salts (such as those containing polyvalent ions, such as Ca ++  and Mg ++ ).

This is a division of application Ser. No. 378,154, filed May 14, 1982,now U.S. Pat. No. 4,526,947.

BACKKGROUND OF THE INVENTION

After using conventional pumping techniques very large amounts of oil ina given reservoir remain unrecovered. In an attempt to recover this vastquantity of unpumped petroleum many enhanced oil recovery (EOR)techniques have been developed. The water flooding method is a verycommon EOR technique that has been in use for some time. Water floodingis a secondary oil recovery technique that is chiefly of importance whenthe natural production of a well has ceased--that is, when petroleum canno longer be pumped from the well economically using conventionalpumping techniques. The term "secondary recovery" as used herein, refersto all petroleum recovery operations used in such areas when spontaneousproduction of the well can no longer be effected. It includes what issometimes known in the industry as "tertiary recovery," which is a laterstage which begins when the petroleum reservoir is substantially"flooded out" and a large amount of water may be produced before any oilis recovered. Thus, primary recovery is when a well spontaneously flowsusing conventional pumping techniques and secondary recovery begins whenprimary recovery is no longer feasible and continues for as long asthere is any petroleum in the well which can be economically or feasiblyremoved.

The water flooding technique comprises injecting water into a petroleumdeposit through at least one input well, thereby causing the petroleumto flow from that area for collection through at least one output well.In the simplest recovery method a number of wells are drilled on thecircumference of a circle and a final well is drilled in the center.Water is then pumped into one or more of the wells, typically the oneson the circumference (sometimes referred to herein as "injectionwells"), under high pressure and forced through the petroleum-bearingformations, usually porous rock strata. The petroleum remaining in thestrata is forced out by the oncoming water and removed through theoutput well, usually the one at the center of the circle. More typicallyan array of injection and production (output) wells are established overan oil field in a manner that will optimize this secondary recoverytechnique by taking into account the geological aspects of thatparticular field.

Ideally, the water should displace 100 percent of the petroleum in theoil field. Even though water may pass through a deposit, the inherentincompatibility of oil and water, variation in reservoir rock, includingpermeability variation, faults and shale barriers may result in someregions of the reservoir rock being by-passed so that large oil bearingareas in the deposit remain untouched. This results in less than 100percent of the residual oil in the reservoir being recovered. Theability of water, or any other fluid, to displace oil is related to thatfluids mobility ratio. Every fluid has a specific mobility in an oildeposit, which can be defined as the ease with which that fluid flowsthrough a porous medium divided by the viscosity of that fluid. Amobility ratio is the ratio of the mobility of two fluids; for example,oil and water. If a fluid flows much more easily than oil through areservoir, it will readily bypass oil deposits within the reservoirrather than pushing them toward producing wells. Thus, fluids with lowmobility ratios are greatly preferred for enhanced oil recoveryapplications. Recovery by water flooding techniques is greatlyfacilitated if the mobility of the petroleum relative to the injectionwater is at a maximum. This is frequently accomplished by increasing theviscosity of the aqueous medium and decreasing the viscosity of thepetroleum, by the addition of suitable chemical agents. Thus, athickener is ordinarily added to the water while a thinning agent may becharged into the petroleum.

High molecular weight (above about 1,000,000) water soluble polymers aregenerally added to the injection water used in EOR applications toimprove the mobility ratio of the water to the oil. A very largeincrease in water viscosity can be obtained when certain polymers areadded in minor amounts (100 ppm to 1500 ppm). Two general types ofpolymers are currently being used for this application, they arepolyacrylamides and polysaccharides. In general, partially hydrolyzedand anionic polyacrylamides are used, but cationic polyacrylamides havealso been used in a limited number of cases. The mobility ratioimprovement obtained using polyacrylamides decreases with water salinityand divalent ion concentration. Therefore, a fresh water source (totaldissolved solids less than 10,000 ppm) has traditionally been necessaryfor the effective use of polyacrylamides in EOR applications asviscosifiers. The environment into which the polyacrylamide solution isinjected must also be substantially free of salts in order to beeffective.

SUMMARY OF THE INVENTION

Poly-N,N-dimethylacrylamide (poly-DMA) is nonionic which makes itinsensitive to metal salts. ##STR1##

Traditionally, low molecular weight has prevented poly-DMA synthesizedby utilizing conventional techniques from providing the high viscosityrequired for EOR applications. This invention presents a technique forthe synthesis of ultra-high molecular weight DMA copolymers (polymerscontaining chain linkages (repeat units) derived from DMA monomer).These high molecular weight DMA copolymers have excellent properties asviscosifiers for EOR applications. This technique utilizes acopolymerization of N,N-dimethylacrylamide monomer (DMA) with sodiumstyrene sulfonate monomer (SSS) or N-methylolacrylamide monomer (NMA).##STR2## An ammonium persulfate/sodium metabisulfite redox initiatorsystem can be utilized in this polymerization. This copolymerizationproduces a polymer with a much higher molecular weight that can besynthesized using DMA monomer alone.

This invention discloses a method for recovering petroleum from asubterranean petroleum bearing deposit which comprises injecting viscouswater into the area of said deposit through at least one input well,thereby causing said petroleum to flow from said area for collectionthrough at least one output well, the improvement which comprisesinjecting said viscous water containing a water soluble polymer withchain linkages derived from N,N-dimethylacrylamide and at least onemember selected from the group consisting of N-methylolacrylamide andsodium styrene sulfonate into the area of said deposit; a method forrecovering petroleum from a subterranean petroleum bearing deposit whichcomprises injecting viscous water into the area of said deposit throughat least one input well, thereby causing said petroleum to flow fromsaid area for collection through at least one output well, theimprovement which comprises injecting said viscous water containing awater soluble polymer with chain linkages derived fromN,N-dimethylacrylamide, at least one member selected from the groupconsisting of N-methylolacrylamide and sodium styrene sulfonate, and atleast one member selected from the group consisting of sodium2-acrylamido-2-methylpropanesulfonate, potassium2-acrylamido-2-methylpropanesulfonate, ammonium2-acrylamido-2-methylpropanesulfonate, and calcium2-acrylamido-2-methylpropanesulfonate into the area of said deposit; aprocess for the synthesis of a high molecular weight polymer whichcomprises: the addition of a metal persulfate and at least one memberselected from the group consisting of sodium metabisulfite, sodiumthiosulfate and sodium dithionite to an aqueous reaction solutionconsisting of N,N-dimethylacrylamide and at least one member selectedfrom the group consisting of N-methylolacrylamide and sodium styrenesulfonate, in amounts and under conditions sufficient to initiate thepolymerization; a process for the synthesis of a high molecular weightpolymer which comprises: the addition of a metal persulfate and at leastone member selected from the group consisting of sodium metabisulfite,sodium thiosulfate and sodium dithionite to an aqueous reaction solutionconsisting of N,N-dimethylacrylamide, at least one member selected fromthe group consisting of N-methylolacrylamide and sodium styrenesulfonate, and at least one member selected from the group consisting ofsodium 2-acrylamido-2-methylpropanesulfonate, potassium2-acrylamido-2-methylpropanesulfonate, ammonium2-acrylamido-2-methylpropanesulfonate, and calcium2-acrylamido-2-methylpropanesulfonate in amounts and under conditionssufficient to initiate the polymerization; a process for the synthesisof a high molecular weight polymer which comprises: the addition of ametal persulfate and at least one member selected from the groupconsisting of sodium metabisulfite, sodium thiosulfite, and sodiumdithionite to a reaction solution comprising water, an oil, dispersingagent, N,N-dimethylacrylamide, and at least one member selected from thegroup consisting of N-methylolacrylamide and sodium styrene sulfonate inamounts and under conditions sufficient to initiate the polymerization;an aqueous polymer solution comprising: water and a high molecularweight polymer with chain linkages derived from N,N-dimethylacrylamideand at least one member selected from the group consisting ofN-methylolacrylamide and sodium styrene sulfonate; and a high molecularweight water soluble polymer comprised of chain linkages derived fromN,N-dimethylacrylamide and at least one member selected from the groupconsisting of N-methylolacrylamide and sodium styrene sulfonate.##STR3##

DETAILED DESCRIPTION

An ultra-high molecular weight copolymer of DMA and SSS can besynthesized in an aqueous medium over a very wide temperature range. Themonomer charge concentration used in an aqueous solution synthesis ofDMA/SSS copolymers can be varied over a wide range from as low as about2 weight percent to as high as about 50 weight percent of the totalreaction solution (monomers, water, initiators, etc.). Generally, it ispreferred to use a monomer charge concentration (total concentration ofall monomers in the aqueous reaction solution) in the range of 10 to 20weight percent. For example, 80 parts of water, 19.8 parts of DMA and0.02 part of SSS (20 weight percent monomer charge concentration) can beemployed in the polymerization recipe utilized in the synthesis ofultra-high molecular weight copolymers of DMA and SSS. In DMA/SSScopolymerizations SSS monomer charge level ranging from about 0.1 toabout 5 weight percent based upon total monomers used in the reactionsolution can be employed. Good results have been obtained using an SSSmonomer charge level from about 0.5 to about 1.5 weight percent basedupon total monomers. Excellent results have been obtained by utilizing atotal monomer concentration of about 20 weight percent (the totalconcentration of all monomers in the reaction solution) in these aqueouscopolymerizations.

Ultra-high molecular weight DMA/NMA copolymers can be synthesizedutilizing a reaction solution comprising DMA, NMA, redox initiators andwater. The DMA/NMA monomer charge concentration used in this aqueouspolymerization can vary over a wide range from as low as about 2 percentto as high as about 50 weight percent of the total reaction solution.Generally, it will be preferred to utilize a monomer chargeconcentration ranging from 10 to 20 weight percent of the total reactionsolution. The charge level of NMA used in such a polymerization recipecan range from as low as about 0.1 weight percent to as high as about 5weight percent based upon total monomers in the reaction solution. Verygood results have been obtained and it will be generally preferred touse a charge level of NMA ranging from about 1 to 3 weight percent basedupon total monomers. For example, a reaction solution comprising 80parts of water, 19.4 parts of DMA, and 0.6 parts of NMA will producevery good results on polymerization (NMA charge level of 3 percent byweight based upon total monomer).

These polymerizations that yield ultra-high molecular weight DMA/SSS andDMA/NMA copolymers can generally be initiated by utilizing free radicalinitiators; for example redox initiator systems, such as metalpersulfate and metabisulfite. Potassium persulfate and ammoniumpersulfate have been used with great success as redox initiators whenused in conjunction with sodium metabisulfite. Various metal persulfates(for example sodium and potassium) and ammonium persulfate (hereinafterthe term metal persulfates will be meant to include ammonium persulfate)can be employed as redox initiators when used in conjunction with sodiummetabisulfate, sodium thiosulfate, and sodium dithionite. These redoxinitiator components can be employed at levels from about 0.01 to about0.1 phm (parts per hundred parts monomer). An initiator level of 0.0375phm ammonium persulfate and 0.0375 phm sodium metabisulfite has beenemployed very successfully to initiate polymerizations of this type. Anumber of other initiator systems can also be employed. For example, ametal persulfate used alone at elevated temperatures can initiate thepolymerization of DMA copolymers (DMA/SSS copolymers and DMA/NMAcopolymers).

The temperature range at which these polymerizations can be run is fromabout 5° C. to about 50° C. The preferred temperature range is from 15°C. to 25° C. with good results being obtained at a temperature of 20° C.The reaction time allowed for the polymerization to occur (time periodbetween the initiation of the polymerization and its termination) isgenerally in the range of about 6 to 18 hours. This reaction time canvary with the temperature of the polymerization and the initiator typesand concentration utilized.

Generally, it is desirable to remove dissolved oxygen from these aqueoussolutions before polymerization. This can be accomplished by spargingthe solution with an inert gas or nitrogen before initiating thepolymerization. It may also be desirable to maintain such a spargingwith an inert gas or nitrogen during the initial stages of thepolymerization.

These aqueous polymerizations which yield ultra-high molecular weightDMA/SSS and DMA/NMA copolymers result in the formation of a watersoluble gel-like mass. This water soluble polymer must be dissolved inadditional water in order to be utilized as a viscosifier for EORapplications. These polymers should be dissolved in an appropriateamount of water to provide a polymer concentration that will result inthe desired viscosity for the injection water. Obviously the viscosityof the injection water increases with increasing polymer concentrations.Generally it will be desirable to have an injection water viscosity(Brookfield) of about 2 to about 30 cps (centipoise) for EORapplications.

When preparing these solutions care should be taken so as to preventshear forces from causing molecular fracture in the polymer chains ofthese copolymers. In order to prevent molecular fracture when dissolvingthese polymers in water vigorously mixing, shaking, etc. shouldgenerally be avoided. The occurrence of such molecular fracture inducedby shearing forces can significantly reduce the molecular weight of thepolymer and therefore its usefulness as an EOR viscosifier (viscositieswould be reduced). In order to dissolve these polymers in water theymust be allowed to dissolve over a very long period of time. These ultrahigh molecular weight DMA copolymers are very valuable as EOR injectionwater viscosifiers because they are transparent to salts (the viscosityof their aqueous solutions is unaffected by salt).

Ultra-high molecular weight terpolymers of DMA, NMA or SSS and metalsalts of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) are veryuseful as viscosifiers for EOR applications. Terpolymers of this typehave very high viscosity in fresh water and also maintain excellentviscosities in saline solution. Sodium AMPS (sodium2-acrylamido-2-methylpropanesulfonate), potassium AMPS (potassium2-acrylamido-2-methylpropanesulfonate), ammonium AMPS (ammonium2-acrylamido-2-methylpropanesulfonate) and calcium AMPS (calcium2-acrylamido-2-methylpropanesulfonate) have all been found to be usefulas monomers in the synthesis of these ultra-high molecular weightterpolymers. The DMA charge level used in the synthesis of a terpolymerof this type can range from about 30 percent to as high as 95 weightpercent of the total monomer charge. The NMA monomer charge level usedin the synthesis of these DMA terpolymers can range between about 0.1weight percent to as high as about 10 weight percent of the totalmonomer charge. The amount of metal-AMPS useful in these terpolymerpolymerizations can range from as low as about 4 weight percent to ashigh as 50 weight percent of the total monomer charge. The amount of NMAmonomer needed in this polymerization decreases with increasing levelsof DMA monomer used in the polymerization. The monomer chargeconcentration can be varied from 2 to 50 weight percent of the totalreaction solution. A monomer charge concentration from 10 to 20 percentby weight is preferred. The optimum percentage of the various monomersused in this terpolymer polymerization varies with temperature, thetotal monomer charge concentration, and initiator levels.

The initiators useful in this DMA terpolymer polymerization are the sameas the initiators that were disclosed for use in DMA copolymersynthesis. The initiator concentration that can be used for thisterpolymer polymerization ranges from as low as 0.01 phm to as high asabout 0.05 phm. The preferred initiator concentration range is from 0.02phm to 0.04 phm. A redox initiator system comprising 0.0375 phm ofammonium persulfate and 0.0375 phm of sodium metabisulfate has been usedwith great success in this terpolymer polymerization. Normally, theredox initiator used to initiate the polymerization is added as 0.5weight percent aqueous solution.

The temperature range at which this polymerization can be run alsovaries over a wide range from as low as about 5° C. to as high as about50° C. The preferred temperature range is again from 15° C. to 25° C.

As was the case for DMA copolymer polymerization it is normallyadvantageous to remove dissolved oxygen from the aqueous chargecomposition. This can be accomplished by sparging the aqueouspolymerization recipe with an inert gas (e.g. nitrogen or helium);employing oxygen scavengers (e.g. sodium dithionite); or degassing witha vacuum. The preferred monomer charge composition recipe used in thisterpolymer polymerization is 40 to 50 weight percent DMA, 0.1 to 10weight percent NMA, and 40 to 50 weight percent metal-AMPS. After thispolymerization is completed, which normally takes 6 to 18 hours, theproduct is in the form of a gel-like mass. Yields in such aqueouspolymerizations are essentially quantitative (in excess of 99 percent).The percentage of chain linkages by weight derived from a monomer in apolymer will be equal to the percentage by weight of that monomer in themonomer charge used in the synthesis of that polymer. This material mustbe dissolved in the amount of water required to provide the desiredviscosity for the injection water used in EOR applications. As was thecase with DMA copolymers, care must be taken to prevent molecularfracture in this terpolymer by shear degradation. This would result inthe loss of viscosity for the injection water being treated per unitweight of this DMA terpolymer used. As was the case with DMA copolymersit takes long time periods to dissolve this DMA terpolymer in injectionwater since severe shearing forces must be avoided (for example,vigorous mixing, stirring, shaking, etc.). The viscosity of theinjection water being treated can be controlled by dissolving therequired amount of this DMA terpolymer in the water. The DMA terpolymerfrom this polymerization has an ultra-high molecular weight and willincrease the viscosity of fresh water very dramatically. This DMAterpolymer is affected by saline solutions, but still retains excellentviscosity in salt water. This terpolymer of DMA, NMA and metal-AMPS isan excellent choice as a general purpose viscosifier for EORapplications.

These DMA copolymers and terpolymers can also be synthesized byutilizing a water-in-oil dispersion polymerization. The ultra-highmolecular weight polymers produced by water-in-oil dispersionpolymerization are in the form of a liquid (in contrast to the gel-likemass formed in aqueous polymerizations). This liquid can easily befurther diluted to the desired polymer concentration for use asinjection water for EOR applications. This further dilution can beachieved almost immediately upon mixing with additional water. Theultimate properties of these DMA copolymers and terpolymers produced bywater-in-oil dispersion polymerizations are equivalent to the propertiesof their counterparts produced by aqueous polymerization (they have thesame excellent properties as EOR viscosifiers). Water-in-oil dispersionpolymerization offers a very substantial advantage over aqueouspolymerization in that the ultra-high molecular weight polymers producedcan be easily and rapidly dissolved (further diluted) in the injectionwater.

The water-in-oil dispersion synthesis of DMA/SSS copolymers, DMA/NMAcopolymers, DMA/SSS/metal-AMPS, and DMA/NMA/metal-AMPS terpolymers isrun utilizing the same monomer charge composition, activators, andreaction conditions as is used in the aqueous polymerization synthesisof these ultra-high molecular weight polymers. In water-in-oildispersion polymerization in addition to the reagents used in aqueouspolymerization, there is also present an oil and a dispersing agent.Some representative examples of oils that can be used are kerosene,diesel fuel, pentane, hexane, decane, pentadecane, benzene, toluene,2,4-dimethylhexane, mineral oil (liquid petrolatum), and 3-ethyloctane.This is certainly not an exhaustive list of the oils that can beemployed. Most alkanes containing 5 or more carbon atoms will work verywell as will most aromatic hydrocarbons. Alkenes should not be usedsince they can react in the polymerization. The dispersing agents arenonionic surfactants that are soluble in hydrocarbons and insoluble inwater. Some representative examples of dispersing agents that may beused in water-in-oil dispersion polymerization include polyethers, suchas Igepal CO-430® (GAF Corp.); polyglycerol oleates, such asWitconol-14® (Witco Chemical Company); and polyglycerol stearates, suchas Witconol-18L® (Witco Chemical Company) and mixtures of these agents.##STR4## These dispersing agents (nonionic surfactants) are added to theoil that will be used in the water-in-oil dispersion polymerization.Normally, the oil used in such dispersion polymerizations will containfrom about 2 to about 20 weight percent of the dispersing agent.Normally, the charge composition used in these water-in-oil dispersionpolymerizations will contain 25 weight percent of the oil containing thedispersing agent in relation to the total reaction solution. Even moreoil can be used in such water-in-oil dispersion polymerization with acorresponding increase in the amount of dispersing agent used butgenerally it will not be advantageous to use larger amounts of the oil.Good results have been obtained using a reaction mixture comprisingabout 25 weight percent monomers, about 50 weight percent water, andabout 25 weight percent oil. A charge composition containing less than25 weight percent monomers can be used, however, it will not beadvantageous to use lesser quantities of the monomers.

It is often desirable to use deionized water in such chargecompositions. Oxygen which is dissolved in the monomers, water, and oilshould be removed before polymerization. This can be accomplished bysparging the monomers, water, and oil with an inert gas or nitrogen.Such a mixture of monomers, water, and oil is vigorously mixed to obtainthe water-in-oil dispersion. The dispersion is brought to the desiredtemperature (normally ambient temperature about 20° C.) and theinitiator components are added. The addition of ammonium persulfatefollowed by sodium metabisulfite has been used with good success as aninitiator. The reaction mixture containing the initiator is normallystirred during the course of the polymerization.

After the desired reaction time the polymerization can be terminated byadding a shortstopping agent, such as methylether hydroquinone.Normally, this reaction time will be from about 6 to about 18 hours. Thedesired reaction time will vary with reaction temperature, initiatorconcentration, and the degree of polymerization desired. Normally, itwill be desirable to allow the polymerization to go to completion (untilthe monomer supply is essentially exhausted). In such water-in-oildispersion polymerizations yields are essentially quantitative (inexcess of 99 percent). The percentage of chain linkages by weightderived from a monomer in a polymer will be equal to the percentage byweight of that monomer in the monomer charge used in the synthesis ofthat polymer.

The performance of these enhanced oil recovery polymers as injectionwater viscosifiers is determined partly by their molecular weight. It isnecessary for these polymers to have a high molecular weight (typically,1,000,000 or greater) to be effective in EOR applications.Determinations of molecular weight are therefore an important aspect inthe characterization procedure of polymeric EOR injection waterviscosifiers. Low angle laser light scattering is a technique that canbe used to determine the weight average molecular weight of thesepolymers. A review of light scattering procedures is presented inJordan, R. C. "Size Exclusion Chromatography With Low Angle Laser LightScattering Detection," Journal of Liquid Chromatography, Vol. 3, No. 3,pp. 439-463 (1980); Tanford, Charles, Physical Chemistry ofMacromolecules (N.Y., John Wiley & Sons, Inc. 1961) pp. 275-316 and pp.390-412; Huglin, M. B., Light Scattering From Polymer Solutions (N.Y.,Academic Press, 1972) pp. 165-203, 291, and 306-331. All of theforegoing references are hereby incorporated by reference in theirentirety.

One analytical method which can be used to determine weight averagemolecular weight is as follows: solutions of dimethylacrylamide/sodiumstyrene sulfonate copolymers (DMA/SSS) were prepared by accuratelyweighing 0.3-0.5 grams of polymer in a tared 100 milliliter volumetricflask. About 75 milliliters of distilled water was added to each of 4flasks and six days were allowed for dissolution. The polymer solutionsin the flasks were then further diluted to volume (100 ml) withdistilled water. All other sample concentrations used in the lightscattering procedure were volumetrically prepared from these solutions.Specific refractive index increments were measured on a Brice-Phoenixdifferential refractrometer equipped with a mercury vapor light sourceand band pass filters of 633, 546 and 436 nm (nanometers). Calibrationwas accomplished with potassium chloride solutions. Low angle laserlight scattering measurements were performed with a Chromatix KMX-6 lowangle laser light scattering photometer, after filtering the solutionsthrough a 1.2 micron filter. All sample scattering was measured usingthe 6°-7° annulus and 0.2 mm field stop The KMX-6 laser has a wavelengthof 633 nm.

The weight average molecular weights of various DMA/SSS copolymers asdetermined by this procedure are given in Table I. These copolymersdiffer in the percent by weight of sodium styrene sulfonate based upontotal monomers in the reaction solution used in their synthesis. TheBrookfield viscosities of these DMA/SSS copolymers were also determinedusing the technique described in Example 3.

                  TABLE I                                                         ______________________________________                                        Weight Average Molecular Weight of DMA/SSS Copolymers                         % SSS     Brookfield Viscosity                                                                        Molecular Weight                                      ______________________________________                                        0.23      3.8           1,760,000                                             0.40      4.4           1,830,000                                             0.60      5.4           2,960,000                                             0.80      6.8           3,590,000                                             ______________________________________                                    

All of these DMA/SSS copolymers have a high molecular weight (in excessof 1,000,000). As can be determined by examining Table I, molecularweight increases with increasing Brookfield viscosities. Various DMA/SSSand DMA/NMA copolymers have been synthesized that exhibit much greaterBrookfield viscosities than those cited in this example and theirmolecular weights would therefore be greater than those determinedabove.

Nuclear magnetic resonance (n.m.r.) spectroscopy was used to confirm theincorporation of chain linkages derived from N,N-dimethylacrylamide andN-methylolacrylamide into DMA/NMA copolymers. These copolymers withchain linkages derived from DMA and NMA can be represented by theformula: ##STR5## wherein x and y are integers and wherein

indicates that the distribution of chain linkages derived from DMA andNMA in the polymer chain is random. Nuclear magnetic resonancespectroscopy was also used to confirm the incorporation of chainlinkages derived from N,N-dimethylacrylamide, N-methylolacrylamide, andsodium-2-acrylamido-2-methyl-propanesulfonate into DMA/NMA/Na-AMPSterpolymers which can be represented by the formula: ##STR6## wherein x,y and z are integers and wherein

indicates that the distribution of chain linkages derived from DMA, NMA,and Na-AMPS in the polymer chain is random.

The DMA/NMA copolymer and DMA/NMA/Na-AMPS terpolymer samples used inthis n.m.r. analysis were synthesized in the 10 mm n.m.r. sample tubesused by adding known amounts of each monomer or its aqueous solution(see Table II) to the tubes, diluting with D₂ O until the total weightof each solution was 2.5 grams, and adding the initiators to polymerizethe samples. The polymerized samples were run on a 20 MHz ¹³ C probewith a gated wide band 'H decoupling sequence. The other instrumentsettings were as follows: sweep width 5000 Hz, acquisition time 0.5second, pulse delay 4.5 seconds, and pulse width 12 micro-seconds.

Table II shows the various components used in the reaction solutionsutilized in the synthesis of these polymers. In each of these samplesthe polymerization was initiated by the addition of 0.030 mls. of a 1%aqueous solution of (NH₄)₂ S₂ O₈ and 0.030 mls. of a 1% aqueous solutionof Na₂ S₂ O₅. Table II also shows the percentage of each of the monomers(based on total monomers) employed in the reaction solutions. The totalamount of chain linkages by weight derived from each of the variousmonomers which has been incorporated into the polymers is also shown incolumn 3 of Table II.

                  TABLE II                                                        ______________________________________                                                                     % of Chain                                                                    Linkages                                                        % Monomer in  Derived From                                     Monomer Components                                                                           Reaction Solution                                                                           Monomers                                         ______________________________________                                        0.50 g DMA     100% DMA      100% DMA                                         0.50 g Na--AMPS                                                                              100% Na--AMPS 100%                                                                          Na--AMPS                                         0.20 g NMA     100% NMA      100% NMA                                         0.50 g DMA/0.0124 g NMA                                                                      97.6% DMA,    97.2% DMA,                                                       2.4% NMA      2.8% NMA                                        0.25 g DMA/0.25 g-                                                                           47.6% DMA, 4.8%                                                                             47.6% DMA,                                       Na--AMPS/0.025 g NMA                                                                         NMA, 47.6%     3.6% NMA,                                                      Na--AMPS      48.8%                                                                         Na--AMPS                                         ______________________________________                                    

The percentage of chain linkages derived from each of these monomers (asshown in column 3) was determined by using this n.m.r. technique. As canbe determined by comparing the percentage of a given monomer in thereaction solution and the percentage of chain linkages derived from thatmonomer in the polymer synthesized, the percentage of chain linkages byweight derived from a monomer in a polymer is essentially equal to thepercentage by weight of that monomer in the reaction solution (basedupon total monomers) used in the synthesis of that polymer. Thesepolymerizations have yields which are essentially quantitative with allof the monomers in the reaction solution being polymerized into thepolymers. The three homopolymers were run as standards and to determinethe chemical shift for the chain linkages derived from each of thesemonomers.

DMA/SSS copolymers which have chain linkages derived fromN,N-dimethylacrylamide and sodium styrene sulfonate can be representedby the formula: ##STR7## wherein x and y are integers and wherein

indicates that the distribution of chain linkages derived from DMA andSSS in the polymer chain is random. DMA/SSS/Na-AMPS terpolymers whichhave chain linkages derived from N,N-dimethylacrylamide, sodium styrenesulfonate, and sodium 2-acrylamido-2-methylpropane sulfonate can berepresented by the formula: ##STR8## wherein x, y, and z are integersand wherein

indicates that the distribution of chain linkages derived from DMA, SSS,and Na-AMPS in the polymer chain is random.

Terpolymers which have chain linkages derived from DMA, NMA, and SSS arealso useful as viscosifiers for EOR applications and can be representedby the formula: ##STR9## wherein x, y, and z are integers and wherein

indicates that the distribution of chain linkages derived from DMA, NMA,and SSS in the polymer chain is random. Polymers that have chainlinkages derived from DMA, NMA, SSS and Na-AMPS are also useful as EORinjection water viscosifiers and can be represented by the formula:##STR10## wherein w, x, y, and z are integers and wherein

indicates that the distribution of chain linkages derived from DMA, NMA,SSS, and Na-AMPS in the polymer chain is random. Other metal-AMPS, suchas K-AMPS, NH₄ -AMPS, and Ca-AMPS are also useful as monomers from whichchain linkages can be derived to form polymers that are useful for EORapplications.

Acrylamide has been copolymerized with DMA and NMA to form a terpolymerthat is useful for EOR applications. It has chain linkages derived fromacrylamide, DMA, and NMA which can be represented by the formula:##STR11## wherein x, y, and z are integers and wherein

indicates that the distribution of chain linkages derived from DMA, NMA,and acrylamide is random. Such terpolymers containing chain linkagesderived from acrylamide are inferior to DMA/NMA copolymers since theacrylamide group can be readily hydrolyzed, yielding a product which isnot transparent to salts. The greater the amount of chain linkagesderived from acrylamide incorporated into such a terpolymer, the morelikely it will be sensitive to salts. However, for cost considerationsit may be advantageous to copolymerize a small amount of acrylamide intoEOR polymers (producing polymers containing chain linkages derived fromacrylamide) notwithstanding the fact that they will become moresensitive to salts. Acrylamide can also be copolymerized to formDMA/SSS/acrylamide terpolymers, DMA/NMA/SSS/acrylamide copolymers,DMA/NMA/metal-AMPS/acrylamide copolymers, DMA/SSS/metal-AMPS/acrylamidecopolymers, and DMA/NMA/SSS/metal-AMPS/acrylamide copolymers which areuseful as EOR injection water viscosifiers, but upon hydrolysis of theacrylamide linkage they will become more sensitive to salts.

The polymerizations that have herein been described utilize variousmonomers to form polymers containing chain linkages (repeat units)derived from these monomers. These chain linkages differ from themonomers that they were derived from in that they no longer contain acarbon-carbon double bond (see the preceding formulae for thesepolymers).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is illustrated by the following examples.

EXAMPLE 1

DMA and SSS were added to deionized water to give a monomerconcentration of 10 weight percent. This solution contained 9.9 percentDMA and 0.1 percent SSS. The solution was thoroughly nitrogen sparged toremove any dissolved oxygen. 100 milliliters of this solution was addedto a 4 ounce polymerization bottle. While continuing to nitrogen spargethis solution 0.0375 phm of ammonium persulfate was added in the form ofa 0.5 percent aqueous solution and 0.0375 phm parts of sodiummetabisulfite was added in the form of a 0.5 percent aqueous solution.This solution was well mixed and the polymerization bottle was cappedand allowed to stand in a constant temperature bath at 20° C. for 18hours. This technique resulted in the synthesis of an ultra-highmolecular weight copolymer of DMA and SSS which had a gel-likeconsistency. This ultra-high molecular weight DMA copolymer is onlyslightly ionic in character and therefore is relatively insensitive tosaline solutions. It is an excellent choice as an EOR viscosifier inapplications where salt in the injection water environment traditionallyhave reduced the viscosity of ionic polymer solutions.

EXAMPLE 2

A 20 percent aqueous solution of DMA and NMA was prepared whichcontained 19.4 percent DMA and 0.6 percent NMA. Deionized water wasutilized in the preparation of this solution. This solution was nitrogensparged for one minute employing a coarse fritted glass tube. Ammoniumpersulfate (0.0375 phm) and sodium metabisulfite (0.0375 phm) wereemployed as 0.5 percent aqueous solutions to initiate thispolymerization. The ammonium persulfate/sodium metabisulfite initiatorwas added while maintaining a nitrogen purge above the solution. The 4ounce polymerization bottle was capped, vigorously shaken, and allowedto stand at 20° C. in a constant temperature bath for about 18 hours.

An ultra-high molecular weight copolymer of DMA and NMA was formed,which had the consistency of a thick gel. This polymer has excellentproperties as an EOR viscosifier for injection water because it is notaffected by saline solution (salt does not decrease the viscosity of theinjection water that has been treated with this copolymer).

EXAMPLE 3

In order to determine the effectiveness of DMA/SSS copolymers andDMA/NMA copolymers as viscosifiers for EOR applications the Brookfieldviscosity of the dilute solutions of the ultra-high molecular weightcopolymers synthesized in Examples 1 and 2 were determined. A 0.3percent solution of the DMA/SSS copolymer (synthesized in Example 1) anda 0.3 percent solution of the DMA/NMA copolymer (synthesized in Example2) were prepared by diluting their respective gel-like masses withdeionized water. The samples were shaken intermittently over a one weekperiod until the material was totally dissolved in the water. Brookfieldviscosities were run at 60 rpm using the number 1 spindle. Thisprocedure was repeated on a separate aliquot adding sufficient NaCl toyield a 3.5% solution and again repeated on another aliquot addingsufficient sea salt to yield a 5.0 percent solution of sea salt. Theresults of this experiment are shown in Table III.

                  TABLE III                                                       ______________________________________                                        Polymer    Salt Free  3.5% NaCl 5.0% Sea Salt                                 ______________________________________                                        DMA/NMA    21.7       19.1      20.2                                          DMA/SSS    25.2       16.5      17.7                                          ______________________________________                                    

The sea salt used in this Example was a synthetic composition composedof 77.76 parts NaCl, 10.88 parts MgCl₂, 4.74 parts MgSO₄, 3.6 partsCaSO₄, 2.46 parts KCl, 0.24 parts KBr, and 0.34 parts CaCO₃.

As can be determined by examining Table III, the copolymer of DMA andSSS was only slightly affected by the saline solution which isexemplified by the slight decrease in viscosity that was observed uponaddition of salt. The decrease in viscosity in the solution of thecopolymer of DMA and NMA was insignificant. Solutions of this DMA/NMAcopolymer are unaffected by saline solutions; the slight decrease inviscosity that was observed can be attributed to further dilution of thepolymer solution by addition of the salt rather than the sensitivity ofthe ultra-high molecular weight polymer to the salt. DMA/NMA copolymersare superior as injection water viscosifiers for use in high salinityenvironments.

EXAMPLE 4

In order to demonstrate the superiority of these DMA copolymers asviscosifiers for high salinity applications they were compared with acommercially available EOR viscosifier, Dow-Pusher 500® (Dow Chemical).It is an ultra-high molecular weight partially hydrolyzedpolyacrylamide. A 0.3 percent solution of Dow-Pusher 500® was preparedand the synthetic sea salt composition described in Example 3 was addedto give a salt concentration of 5 percent. The Brookfield viscosity ofthis solution was determined using the technique described in Example 3.The Dow-Pusher 500® provided a Brookfield viscosity of 10.1 cps. TheBrookfield viscosity determined for the same concentration of theDMA/NMA copolymer in the same sea salt solution was twice as high (seeExample 3). Since in practice salts are often present in injection waterand the subterranean regions where this water is injected, theseultra-high molecular weights DMA copolymers offer a very distinctadvantage over present injection water viscosifiers.

EXAMPLE 5

Na-AMPS was prepared by the stoichiometric addition of AMPS powder to aNaOH solution. The pH of this solution was adjusted to between 9 and 10by the addition of AMPS or dilute NaOH; this solution was diluted withdeionized water to yield a 20 weight percent solution. This solution waskept at a temperature between 5° and 15° C. during the reaction and thepH of this solution was maintained above 9. AMPS was added until the pHof the solution reached 9. More sodium hydroxide could have been addedto keep the pH above 9 and allow for the addition of more AMPS. Thereaction product of this procedure was sodium-AMPS and water.

1.12 grams of a 14.3 percent solution of NMA in DMA was added to a 8dram (29.57 ml.) glass vial followed the addition of 1.92 grams of a 50percent aqueous solution of Na-AMPS (prepared using the proceduredescribed above). This solution was further diluted to 10 grams totalweight (the total weight of the water and monomers in the solution) withdeionized water to obtain a 20 percent monomer concentration. Thesolution was nitrogen sparged for a period of 4 minutes while the vialwas immersed in a 0° C. constant temperature bath. 0.1 ml. (milliters)of a 0.5 percent solution of ammonium persulfate was added. This wasfollowed by the addition of 0.1 milliliters of a 0.5 percent solution ofsodium metabisulfite. The vial was capped, shaken and placed in a 10° C.constant temperature bath for a period of 18 hours. This polymerizationyields an ultra-high molecular weight DMA terpolymer where thepolymerization mass has a gel-like consistency.

Aliquots of this polymerization mass were diluted to concentrations of0.25 percent in salt free water and salt water of varyingconcentrations. The Brookfield viscosity for these solutions wasdetermined using the technique described in Example 3. The Brookfieldviscosity for these solutions is shown in Table II.

                  TABLE II                                                        ______________________________________                                        EFFECT OF SALT ON THE VISCOSITY OF                                            DMA/Na--AMPS/NMA Terpolymers                                                  Salt Concentration (ppm)                                                                       Brookfield Viscosity (cps)                                   ______________________________________                                          0              4820.0                                                        1000            295.0                                                         5000            59.0                                                         10000            43.0                                                         20000            30.5                                                         50000            26.5                                                         100000           23.3                                                         ______________________________________                                    

The salt composition used in this Example had a composition of 75percent sodium chloride and 25 percent calcium chloride. As is readilyapparent from examining Table II, this terpolymer has an ultra-highviscosity in fresh water and very respectable viscosity in high salinitywater. It has excellent characteristics as a general purpose viscosifiersuitable for use in both fresh and high salinity water. It also hasexcellent thermal stability and stability in the presence of divalentions (in the presence of Ca++ ions there is no precipitation).

EXAMPLE 6

The procedure specified in Example 5 was utilized to synthesize aterpolymer of DMA, NMA, and K-AMPS except that potassium hydroxide wassubstituted for the sodium hydroxide. The Brookfield viscosity for thesolution that was prepared was determined in the same manner that wasspecified in Example 3. A Brookfield viscosity of 20.5 cps was observedat a concentration of 0.25 percent of this terpolymer in a 10 percentaqueous sodium chloride solution. This proves that K-AMPS can besubstituted for Na-AMPS with great success to produce terpolymers of DMAthat are very useful as EOR viscosifiers.

EXAMPLE 7

Using the process described in Example 6 ammonium hydroxide wassubstituted for potassium hydroxide to form a terpolymer of DMA, NMA andammonium-AMPS. Using the process described in Example 3 the Brookfieldviscosity for this terpolymer in a 10 percent salt solution (75 partsNaCl and 25 parts CaCl₂) was determined to be 11.0 cps. This is anexcellent viscosity for a solution that is this high in salinity.

EXAMPLE 8

120 grams of a 33 percent solution of DMA in deionized water was addedto an 8 ounce polymerization bottle fitted with a self-sealing gasketand Teflon® liner (Teflon is a trademark of duPont). 3.33 grams of a 48percent solution of NMA in deionized water was added to the solution andwas nitrogen sparged for 10 minutes. 60 milliters of a 6 percentsolution of Igepal CO-430 (dispersing agent) in hexane solution(previously nitrogen sparged) was added under a nitrogen atmosphere.This mixture was vigorously agitated. 4.4 milliliters of a 0.5 percentaqueous solution of ammonium persulfate was added by injection with asyringe, followed by 4.4 milliliters of a 0.5 percent aqueous solutionof sodium metabisulfite. This polymerization was terminated after sixhours by the addition of 2 milliliters of a 2 percent aqueous solutionof methylether hydroquinone.

The Brookfield viscosity of a 0.25 percent aqueous solution of thispolymer was determined to be 16 cps in a 10 percent salt solution (75parts NaCl and 25 parts CaCl₂) by the method described in Example 3. Theproduct formed in this water-in-oil dispersion polymerization of DMA andNMA is a liquid which can easily be further diluted in water to form ahomogeneous solution. This is in contrast to the thick gel-like materialthat is formed in aqueous polymerization of DMA/NMA copolymers, whichrequire long time periods in order to be further diluted with additionalwater. In practice, it is contemplated that water-in-oil dispersionpolymerization will be employed since it will be necessary to furtherdilute these polymers in injection water for EOR applications.

EXAMPLE 9

38 grams of a 33 percent aqueous solution of DMA in deionized water wasadded to an 8 ounce (236.6 ml.) polymerization bottle fitted with aself-sealing gasket and Teflon® liner. 0.125 grams of SSS was added tothis solution and was nitrogen sparged for 10 minutes. 12.5 grams of a 6percent solution of Igepal CO-430 in a hexane solution, which waspreviously nitrogen sparged, was added under a nitrogen atmosphere. 0.8milliliters of a 1.0 percent aqueous solution of ammonium persulfate wasadded by injection with a syringe. This was followed by the addition of0.8 milliliters of a 1.0 percent aqueous solution of sodiummetabisulfite. This mixture was vigorously agitated. This polymerizationwas terminated after 6 hours by the addition of 2 milliliters ofmethylethyl hydroquinone solution. The product of this polymerizationwas a liquid. The Brookfield viscosity of a 0.25 percent aqueoussolution of this DMA/SSS copolymer was determined to be 16 cps by thetechnique described in Example 3.

The polymers described herein can find broad application for enhancingthe recovery of tertiary oil. Many processes are known in the art forutilizing polymers in such recovery. For example, the polymericinjection water viscosifiers described in this invention can be used asmobillity buffers in conventional EOR techniques, such as chemical,miscible and steam. These polymers generally can also be used forreservoir profile improvement through selective adsorption and/orgelation. These techniques are described in more detail in H. K. vanPoollen, Fundamentals of Enhanced Oil Recovery, (Tulsa, Okla., PennWellBooks, 1980), which is incorporated herein by reference in its entirety.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention. Unlessspecifically indicated otherwise, parts and percentages are given byweight.

What is claimed is:
 1. In a method for recovering petroleum from asubterranean petroleum bearing deposit which comprises injecting viscouswater into the area of said deposit through at least one input well,thereby causing said petroleum to flow from said area for collectionthrough at least one output well, the improvement which comprisesinjecting said viscous water containing a water soluble polymer withchain linkages derived from a major amount of N,N-dimethylacrylamide andat least one member selected from the group consisting ofN-methylolacrylamide and sodium styrene sulfonate into the area of saiddeposit, wherein said water soluble polymer has a molecular weight inexcess of 1,000,000 and wherein said water soluble polymer has betweenabout 0.1 and about 5 percent by weight of its chain linkages beingderived from members selected from the group consisting ofN-methylolacrylamide and sodium styrene sulfonate.
 2. A method asspecified in claim 1 wherein said water soluble polymer has between 0.1and 5 percent by weight of its chain linkages being derived fromN-methylolacrylamide.
 3. A method as specified in claim 2 wherein saidwater soluble polymer has 1 to 3 percent by weight of its chain linkagesbeing derived from N-methylolacrylamide.
 4. A method as specified inclaim 1 wherein said water soluble polymer has between 0.1 and 5 percentby weight of its chain linkages being derived from sodium styrenesulfonate.
 5. A method as specified in claim 4 wherein said watersoluble polymer has between 0.5 and 1.5 percent by weight of its chainlinkages being derived from sodium styrene sulfonate.
 6. A method asspecified in claim 1 wherein a sufficient amount of said water solublepolymer is dissolved in said water to increase the Brookfield viscosityof said water to between as low as 2 cps and as high as 30 cps.
 7. In amethod for recovering petroleum from a subterranean petroleum bearingdeposit which comprises injecting viscous water into the area of saiddeposit through at least one input well, thereby causing said petroleumto flow from said area for collection through at least one output well,the improvement which comprises injecting said viscous water containinga water soluble polymer with chain linkages derived from a major amountof N,N-dimethylacrylamide; at least one member selected from the groupconsisting of N-methylolacrylamide and sodium styrene sulfonate; and atleast one member selected from the group consisting of sodium2-acrylamido-2-methylpropanesulfonate, potassium2-acrylamido-2-methylpropanesulfonate, ammonium2-acrylamido-2-methylpropanesulfonte, and calcium2-acrylamido-2-methylpropanesulfonte into the area of said deposit,wherein said water soluble polymer has a molecular weight in excess of1,000,000 and wherein said water soluble polymer has between about 0.1and about 10 percent by weight of its chain linkages being derived frommembers selected from the group consisting of N-methylolacrylamide andsodium styrene sulfonate.
 8. A method as specified in claim 7 whereinsaid water soluble polymer has 40 to 50 percent by weight of its chainlinkages being derived from N,N-dimethylacrylamide; 40 to 50 percent byweight of its chain linkages being derived from at least one memberselected from the group consisting of sodium2-acrylamido-2-methylpropanesulfonate, potassium2-acrylamido-2-methylpropanesulfonate, ammonium2-acrylamido-2-methylpropanesulfonate, and calcium2-acrylamido-2-methylpropanesulfonate; and 0.1 to 10 percent by weightof its chain linkages being derived from at least one member selectedfrom the group consisting of N-methylolacrylamide and sodium styrenesulfonate.
 9. A method as specified in claim 7 wherein the only memberselected from the group consisting of sodium2-acrylamido-2-methylpropanesulfonate, potassium2-acrylamido-2-methylpropanesulfonate, ammonium2-acrylamido-2-methylpropanesulfonate, and calcium2-acrylamido-2-methylpropanesulfonate is sodium2-acrylamido-2-methylpropanesulfonate.
 10. A method as specified inclaim 7 wherein a sufficient amount of said water soluble polymer isdissolved in said water to increase the Brookfield viscosity of saidwater to between as low as 2 cps and as high as 30 cps.
 11. In a methodfor recovering petroleum from a subterranean petroleum bearing depositwhich comprises injecting viscous water into the area of said depositthereby causing said petroleum to flow from said area for collection,the improvement which comprises injecting said viscous water containinga water soluble polymer with chain linkages derived from major amountN,N-dimethylacrylamide and at least one member selected from the groupconsisting of N-methylolacrylamide and sodium styrene sulfonate into thearea of said deposit, wherein said water soluble polymer has a molecularweight in excess of 1,000,000 and wherein said water soluble polymer hasbetween about 0.1 and about 5 percent by weight of its chain linkagesbeing derived from members selected from the group consisting ofN-methylolacrylamide and sodium styrene sulfonate.
 12. A method asspecified in claim 11 wherein said water soluble polymer has between 0.1and 5 percent by weight of its chain linkages being derived fromN-methylolacrylamide.
 13. A method as specified in claim 12 wherein saidwater soluble polymer has 1 to 3 percent by weight of its chain linkagesbeing derived from N-methylolacrylamide.
 14. A method as specified inclaim 11 wherein said water soluble polymer has between 0.1 and 5percent by weight of its chain linkages being derived from sodiumstyrene sulfonate.
 15. A method as specified in claim 14 wherein saidwater soluble polymer has between 0.5 and 1.5 percent by weight of itschain linkages being derived from sodium styrene sulfonate.
 16. A methodas specified in claim 11 wherein a sufficient amount of said watersoluble polymer is dissolved in said water to increase the Brookfieldviscosity of said water to between as low as 2 cps and as high as 30cps.
 17. In a method for recovering petroleum from a subterraneanpetroleum bearing deposit which comprises injecting viscous water intothe area of said deposit thereby causing said petroleum to flow fromsaid area for collection, the improvement which comprises injecting saidviscous water containing a water soluble polymer with chain linkagesderived from a major amount N,N-dimethylacrylamide; at least one memberselected from the group consisting of N-methylolacrylamide and sodiumstyrene sulfonate; and at least one member selected from the groupconsisting of sodium 2-acrylamido-2-methylpropanesulfonate, potassium2-acrylamido-2-methylpropanesulfonate, ammonium2-acrylamido-2-methylpropanesulfonate, and calcium2-acrylamido-2-methylpropanesulfonate into the area of said deposit,wherein said water soluble polymer has a molecular weight in excess of1,000,000 and wherein said water soluble polymer has between about 0.1and about 10 percent by weight of its chain linkages being derived frommembers selected from the group consisting of N-methylolacrylamide andsodium styrene sulfonate.
 18. A method as specified in claim 17 whereinsaid water soluble polymer has 40 to 50 percent by weight of its chainlinkages being derived from N,N-dimethylacrylamide; 40 to 50 percent byweight of its chain linkages being derived from at least one memberselected from the group consisting of sodium2-acrylamido-2-methylpropanesulfonate, potassium2-acrylamido-2-methylpropanesulfonate, ammonium2-acrylamido-2-methylpropanesulfonate, and calcium2-acrylamido-2-methylpropanesulfonate; and 0.1 to 10 percent by weightof its chain linkages being derived from at least one member selectedfrom the group consisting of N-methylolacrylamide and sodium styrenesulfonate.
 19. A method as specified in claim 17 wherein the only memberselected from the group consisting of sodium2-acrylamido-2-methylpropanesulfonate, potassium2-acrylamido-2-methylpropanesulfonate, ammonium2-acrylamido-2-methylpropanesulfonate, and calcium2-acrylamido-2-methylpropanesulfonate is sodium2-acrylamido-2-methylpropanesulfonate.
 20. A method as specified inclaim 17 wherein a sufficient amount of said water soluble polymer isdissolved in said water to increase the Brookfield viscosity of saidwater to between as low as 2 cps and as high as 30 cps.